On alternating current (ac) power systems, frequency is regulated by maintaining a balance between power generation and power consumption. When generation exceeds consumption for example, the surplus energy is converted to shaft kinetic energy of synchronous turbine generators that are on line, increasing their rotational speed and, hence, the system frequency. Similarly, when consumption exceeds generation, shaft kinetic energy is extracted from system turbine generators and converted to electric power, thereby reducing shaft speed and frequency. Since consumption is generally uncontrollable, frequency is controlled by adjusting generation output to balance demand. Different parts of the world have different rules dictating how this balancing is performed. In most of North America, for example, frequency regulation of electric power utilities is performed in accordance with control performance standards established by North American Electrical Reliability Corporation (NERC). Compliance with such frequency regulation standards is presently measured with parameters, such as Control Performance Standards 1 and 2 (CPS1 and CPS2).
For example, CPS1 is a statistical measure of the Area Control Error and its relationship to frequency error. The Area Control Error (ACE) represents the difference between the actual power interchange and the scheduled interchange, adjusted by a frequency correction. In the aggregate, it represents the difference between actual generation plus external purchases and scheduled generation, which is based on expected demand. Acceptable CPS1 scores can be attained as long as generation operates in a manner to support frequency, i.e., reducing generation output when frequency is above the nominal value (60.0000 Hz in North America) and reducing generation when frequency is low. Wind turbines do not normally operate in this manner. Instead of responding to frequency, wind turbine outputs respond to wind characteristics. At any given time, there is roughly a 50% likelihood that wind power variations will operate in a manner to exacerbate frequency variations. Consequently, it has been observed that increasing proliferation of wind results in declining performance on the NERC CPS1, e.g., in ERCOT (the Electric Reliability Council of Texas), which has observed declining performance on CPS1 as wind penetration levels have increase. Another performance standard is CPS2, which is a statistical measure of unacceptably high or low net generation. Some regions (e.g., Montana) have observed declining CPS2 performance resulting from increased wind penetration levels and resulting high variations in actual power generation compared with scheduled generation. These results are as expected because variations in wind power do not correlate at all with changes in electrical system frequency. Available wind power and system frequency are independent.
Consequently, unlike fossil-based generation, which is generally responsive to system frequency and/or load demands, the randomness of wind power is likely to result in scores that on the average would result on penalties about half of the time. It is possible that any monetary penalties associated with failure to comply with the applicable frequency regulation standards may be substantial and such penalties may be passed on to wind farm owners if it can be shown that non-compliance of frequency regulation results from an inability to appropriately control the output power from the wind turbines. In any event, it is clear that wind turbines do not presently act in a manner to support system frequency on a continuous basis.
It will be appreciated that, although the issues above have been described in the context of an example North American-based frequency regulation standard, the underlying issues apply world-wide to any wind turbine since the random nature of wind knows of no geopolitical boundaries and the methods of frequency control used in the rest of the world are similar to those used in the US and Canada. Therefore there is a need to overcome the above-mentioned issues and to provide apparatus and control techniques conducive to improvements in connection with frequency regulation for wind-driven power generation systems.